Pressure-filtering apparatus and filtering method

ABSTRACT

An apparatus and method for pressure-filtering suspensions inside a filter chamber to separate them into liquor and solids in a continuous operation, the apparatus consisting essentially of two concentrically journalled oppositely rotating filter drums forming an annular filter chamber between their walls, the pressurized suspension being fed into the filter chamber via a feed chamber and an axial, annular feed duct between the drum shafts, the dried product cake being similarly discharged via an axial, annular discharge duct between the drum shafts and a discharge chamber. The apparatus may include a washing zone in the filter chamber being supplied with pressurized washing liquid through the inside of the inner filter drum.

Titus [4 1 Mar. 19, 1974 PRESSURE-FILTERING APPARATUS AND FILTERINGMETHOD [76] Inventor: Hans-Joachim Titus,

Von-l-lees-Strasse 5, 6148 Heppenheim, Germany [22] Filed: Nov. 8, 1971[21] Appl. No.: 196,370

[30] Foreign Application Priority Data Nov. 9, 1970 Germany 2054968 [52]US. Cl 210/78, 210/342, 210/360, 210/394, 210/403 [51] Int. Cl B0ld21/26 [58] Field of Search 210/78, 81, 203, 211, 217, 210/326, 342, 380,394, 403, 329, 360, 360 A 1,218,903 5/1960 France 210/385 PrimaryExaminer-Frank A. Spear, Jr. Assistant Examiner-F. F. Calvetti Attorney,Agent, or Firm-Bacon & Thomas [5 7 ABSTRACT An apparatus and method forpressure-filtering suspensions inside a filter chamber to separate theminto liquor and solids in a continuous operation, the apparatusconsisting essentially of two concentrically journalled oppositelyrotating filter drums forming an annular filter chamber between theirwalls, the pressurized suspension being fed into the filter chamber viaa feed chamber and an axial, annular feed duct between the drum shafts,the dried product cake being similarly discharged via an axial, annulardischarge duct between the drum shafts and a discharge chamber. Theapparatus may include a washing zone in the filter chamber beingsupplied with pressurized washing liquid through the inside of the innerfilter drum.

17 Claims, 2 Drawing Figures PATENTEDMAR 19 I974 SHEET 1 [1F 2 wilPRESSURE-FILTERING APPATIJS A IFILTERING METHOD BACKGROUND OF THEINVENTION 1.,Field f the Invention The present invention relates tofiltering devices and methods, and in particular to pressure-filteringdevices in which filterable suspensions are separated in a filterchamber under pressure into liquor and solids.

2. Description of the Prior Art In the prior art of filtrationtechnology are known several filtering methods and a number of devicesfor the industrial application of these methods. For easily filterablesuspensions, one normally uses a filter centrifuge to separate them intoliquor and solids. However, for suspensions which are more difficult tofilter, one has to resort to filter presses, vacuum filters, or rotaryfilters.

The known filtering devices, almost without exception, have theshortcoming that they require batch operation because they cannot beoperated continuously; many of them are not gastight; and the low upperlimits on the applicable filtering pressures frequently giveunsatisfactory results in terms of liquor removal.

In the case of filter presses in particular, several workers have to beassigned to remove the product from the filter frame after terminationof filtration, to meticulously clean each chamber, and to prepare thefilter press for the next load. Frequently it is also necessary to usewater to wash the device, while a solvent would be quicker and moreeffective because the vapors of acids and solvents present a hazard tothe operating personnel.

Filter presses have the additional disadvantage that, even though theymay be charged at a nominal pressure of 15 atmospheres, the effectivepressure on the cake is much lower. As the suspension is pressed intothe filter chambers, the product initially deposits as a layer on thefilter cloth. The product layer then grows steadily in thickness, sothat the liquor of the later entering suspension has to be squeezedthrough the already compacted layer of product cake, therebyprogressively reducing the effectiveness of the device. Aftertermination of pressure filtering and release of the pressure, thefilter cake develops cracks and openings through which the washingliquid passes with relatively little effect and without washing thecake. For this reason, it may frequently be necessary to wash theproduct for as long as 24 hours in order to remove all acid residues.Thus, the inadequacies of the device have to be compensated by anexcessive consumption of liquid and by a lengthy operation.

Lastly, the degree of liquor extraction and concentration of productachievable with known filter presses is inadequate, because thepressures to be used are severely limited (maximum l at. As a result ofthis limitation, the products obtained with known pressurefilteringmethods contain still some 50 to 80 percent of liquid. In the case wherethis liquid is the leftover chemical agent which was to be removedthrough filtering, it becomes necessary to wash the agent out of thecake. This in turn requires a costly subsequent operation in which thechemical agent must be reclaimed from the wash water.

SUMMARY OF THE INVENTION It is a primary objective of the presentinvention to provide an improved filtering method and an apparatus forits efficient application, where the following conditions are satisfied:

a. The operation is a continuous one;

b. The range of pressures applicable may vary from vacuum to as high as-250 at.;

c. The entire operation is gas-tight, thus permitting the use ofsolvents for washing without contamination of the air surrounding theapparatus;

d. The apparatus can be operated under sterile conditions, thuseliminating any risk of product adulteration;

e. Filtration itself occurs continuously and in a product layer which ismaintained in controllable turbulence and thereby homogeneouslyintermixed so that no cracks or openings can be created in the cake andno problem exists with respect to filtering of the later enteringsuspension, as no dry cake layer can pack against the filter cloth;

f. While the filtering effect is greatly improved, a minimum of washingliquid is required;

g. No operating personnel is necessary because of the continuousoperation;

h. In comparison to known filtering methods a ten to 30-fold increase inspecific filtering efficiency is obtained;

i. The downtimes for cleaning, product removal and washing areeliminated; and

j. The apparatus requires a minimum of space, while giving a highoutput.

In order to attain the earlier-mentioned objective and to achieve theimproved conditions outlined above, the invention suggests a filteringapparatus which includes a chamber formed by two filter surfaces whichmove relative to one another and which are connected to a liquor outlet.

A preferred embodiment of the apparatus of the invention ischaracterized in that the two filtering surfaces are constituted by twoconcentrically journalled, oppositely rotating cylindrical filter drumswhich between them form an annular chamber into which the suspensionenters on one axial end and from which the compacted product cake exitson the other axial end. A portion of the annular chamber may be arrangedas a wash zone, whereby washing liquid is supplied through the center ofthe apparatus and pressed across that portion of the inner filteringsurface which is located in the wash zone.

In principle, the axis of the apparatus of the invention may be orientedin any direction. In a preferred orientation which is vertical, thesuspension is fed under pressure from underneath to the filter chambervia a central entry chamber which is formed by two rotary seals. Thepressurized suspension advances upwardly between the two rotating filterdrums where the liquor contained in the suspension escapes underpressure across the inner and outer moving filter surfaces.

Both filter drums are preferably rotated in opposite directions so thatturbulence is created through the constant transverse motion of bothfilter surfaces relative to the pressurized product cake, therebyhomogenizing the latter and preventing the formation of a stationary dryproduct layer. It should be understood, of course, that where specialconditions exist it is also possible to maintain one of the two drumsstationary, or to rotate both drums in the same sense at differentspeeds of rotation. In general, however, an optimal homogenization isobtained by rotating both filter drums in opposite directions. When oneof the filter drums is stationary, there exists the possibility withcertain products that the turbulence near the stationary filter surfaceis inadequate to prevent the formation of openings across the driedproduct layer, thus permitting free passage of the washing liquid acrossthe product without washing the latter. Furthermore, the uneven shearresistance and viscosity of the different product layers thus createdmay cause the product cake to discharge from the apparatus at anirregular, pulsing rate.

The rotational drive of each filter drum is preferably independent andcontinuously adjustable. The continuous movement of the two filtersurfaces creates frictional interference between the product cake andthe filter cloth, whereby the dry cake is constantly removed from thecloth and intermixed with those parts of the suspension which are moredistant from the filter surfaces and therefore contain more liquor. Thismethod effectively prevents the creation of a dry cake layer on thefilter cloth. The feed pressure of the suspension is evenly distributedover the entire chamber cross section. As most of the products beingprocessed in this manner have thixotropic characteristics, i.e., havinga structural viscosity, their viscosity is considerably lowered and thecake is kept in a liquid state through the shearing action continuouslyoccuring between the product and the two filter surfaces. In comparison,the viscosity of a stationary cake layer of comparable liquor contentwhich adheres to the filter cloth in considerably higher. Thisliquefying effect on thixotropic substances renders it unnecessary toprovide additional mechanical means for advancing the cake in the axialdirection. On the other hand it is easily possible to provide suchadvancing means on one or both filter drums, by adding for examplehelical ribs or drive wings to create a screw conveyor effect.

The higher feed pressures and the improved distribution of the pressureinside the filter chamber make possible a far more complete removal ofthe liquid agents of such suspensions as culture solutions, for example,where the liquid agent may be retained inside microcapillaries oforganically formed tubular structures. This is made possible because thehigh feed pressure and the shearing action during filtering tend tobreak open these tubular structures, thereby freeing the reaction agent.

BRIEF DESCRIPTION OF THE DRAWINGS Further special features andadvantages of the invention will become apparent from the descriptionfollowing below, when taken together with the accompanying drawingswhich illustrate, by way of examples, several embodiments of theinvention, represented in the various figures as follows:

FIG. 1 shows, in a partially schematic representation, a longitudinalcross section of a filtering apparatus embodying the invention; and

FIG. 2 shows a modified version of the inner filter drum of theapparatus of FIG. 1 including an adjustable wash zone.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The example of apparatus chosenas an embodiment of the invention has a vertically oriented axis, itbeing understood that the operation of the apparatus would beessentially the same in any other orientation.

The apparatus of the invention includes a frame 10, a gas-tight casing11 which carries an upper closure panel 12 and a lower collecting trough13 with an outlet 14 acting as a funnel. In the central axis of thecasing 11 are arranged two oppositely aligned journal housings 15 and16, one mounted on the closure 12 and the other on an intermediatebottom wall above the collecting vessel 13. A self-aligning rollerbearing 17 in the lower journal housing 16 supports an inner hollowshaft 18, while a second self-aligning roller bearing 19 supports anouter, shorter hollow shaft 20. Between them, the two independent hollowshafts form an annular axial feed channel which extends upwardly from anannular feed chamber 21 arranged inside the journal housing 16. Intothis feed chamber 21 is fed the pressurized product suspension via aradial intake pipe 22. The journal housing 16 includes rotary seals 34and 35 at both axial sides thereof to prevent any loss of pressurizedsuspension into the bearing areas or to the outside of the feed chamber.These rotary seals must be of a very reliable type, as feed pressures upto 250 atmospheres may be employed.

The two hollow shafts 18 and 20 are connected to the lower drum flanges23 and 24, respectively, thus extending the diameter of the annularchamber between the two shafts to the diameter of the annular filterchamber 24. All corners are rounded to facilitate the advance of thesuspension and to avoid the formation of deposits.

The outer filter drum 26 consists of a drum wall 28 which is perforatedby a large number of transverse bores and which carries on its innersurface a supporting fabric and a filter cloth. Similarly, theconcentrically spaced inner filter drum 27 has a drum wall 29 withnumerous transverse holes and carries on its outer surface a supportingfabric and a filter cloth. The lower flanges 23 and 24 and similar upperflanges 30 and 31 are removably attached to the end faces of theirrespective drum walls 28 and 29 by a row of clamping screws, forexample. Thus, the filtering surfaces become accessible for inspectionand servicing by lifting the drum assembly and closure panel 12 from thecasing 11 and by removing the clamping screws from the upper outerflange 30, after which the outer filter drum 26 with its lower flange 24and hollow shaft 20 can be removed from the drum assembly, therebygiving convenient access to both filter surfaces.

The liquid leaving the filter chamber radially outwardly drips downwardover splash plates and guide panels (not shown) and along the wall ofthe casing 11, being collected underneath in the collecting trough 13.The liquid leaving the filter chamber radially inwardly also flowsdownward, but inside the hollow shaft 18 from where it discharges intothe collecting trough 13. The drum 27 and the inner lower flange 23 mayin addition be provided with guide ribs whose purpose is to force theliquid downward against the centrifugal force which tends to retain itabove the lower flange 23. In most cases, however, the pressure appliedto the suspension is so high that even the centrifugal forces created byhighspeed rotation can be safely neglected.

The upper end of the drum assembly is similar to the lower one in that ahollow outer shaft 32 and an inner shaft 33 extend upwardly from theirrespective flanges 30 and 31 into the upper journal housing 15. Betweenthem the shafts 32 and 33 form a continuation of the annular channelwhich opens into an annular discharge chamber 36 which is similar to thefeed chamber 211. The two shafts are again supported by self-aligningroller bearings 17a and 19a, and the discharge chamber 36 is sealed offaxially in both directions by rotary seals 34a and 350. A discharge pipe37 extends radially from the discharge chamber 36, and a valve 38 in thedischarge pipe 37 can be used to adjust the counter pressure to the feedpressure in order to pressurize the suspension, especially duringstartup of the filtering operation.

The inner shaft 33 extends axially beyond the upper journal housing andcarries on its free portion a pinion or a pulley 39 which, via atransmission drive 40, receives its motive power from an adjustabledrive 41. The outer shaft also carries a pulley 42, arranged below thejournal housing 15, which, via a transmission 43, is similarly connectedto an adjustable drive 44. Because the outer shaft drive 42, 43 islocated within the gastight casing, the drive shaft 45 of itstransmission includes a rotary seal where it passes through the closureplate 12. With this arrangement the two filter drums 27 and 28 arecompletely independent from one another, and their direction of rotationas well as their speed can be adjusted continuously and independently.

ln order to increase the concentration of the product and the rate ofreclamation of the liquid reaction agent, the product discharged throughthe pipe 37 may be recirculated once or several times by returning it tothe feed pipe 22. it is also possible to add washing liquid to therecirculating product, which permits adjustment of an optimal viscosityand consistency for the filtering operation.

Apart from the abovedescribed possibility of recirculation, it is alsopossible to arrange a washing step in between filtering operationsinside the filter chamber so that, during the continuous operation, thesuspension is initially filtered, then washed, and subsequently filteredagain as it passes upwardly through the filter chamber between the drums26 and 27. For this purpose the apparatus as shown in FIG. 1 is providedwith a wash zone (not shown) in an intermediate portion of the length ofthe inner filter drum 27. Such a wash zone can be created by arrangingtwo axially spaced separation walls inside the drum wall 29 which rotatetherewith. To the lower one of the two separation walls is connected acentral pipe 47 through which pressurized washing liquid is suppliedinto the wash zone. The central pipe 47 is arranged inside the hollowinner shaft 13, rotating therewith but leaving a flow space between itand the shaft for the collecting liquor. On its lower end the rotatingpipe 47 passes through a rotary seal into a connecting chamber 47aarranged below the collecting trough l3, and from there a stationarypipe leads to the outside of the apparatus.

In most filtering procedures the washing step is of major importance,and the filtering apparatus of the invention offers the possibility ofcontinuously washing the product inside the filtering zone under highpressure and under intense movement of the product. One

variant of a known washing procedure consists in plastifying the driedproduct cake in an agitator vessel and then again filtering the product.With the oppositely rotating filter drums of theinvention it is possibleto obtain such plastification in a most simple way during the travel ofthe product between the two filter drums. To achieve this, it may beindicated to have no perforations in the outer drum wall 23 in thelength portion of the wash zone. Longitudinal adjustment of the washzone can be provided by having a smooth cylindrical inner diameter onthe inner filter drum 27, the position of the separation walls beingdetermined by repositionable retaining rings and the walls being sealedagainst the drum surface by O-rings. In this case it is necessary tomake the central tube 47 axially adjustable relative to the inner hollowshaft 118.

in FIG. 2 is shown a modified version of the inner filter drum 27 whichincludes a wash zone which is adjustable in its axial position as wellas in its length. For this purpose the inner diameter 50 of the drum 27is tapered outwardly from its longitudinal midpoint in both axialdirections, the taper angle as shown in the drawing being exaggeratedfor purposes of illustration. The end plates 51 and 52 of the wash zoneare axially positioned by means of a central tube 53 with threadedclamping collars which hold them sealingly against the drum tapers 50.For a different length or axial position of the wash zone within thedrum 27 it is necessary to replace the end plates 51 and 52 with similarplates of a different diameter. When the two end plates have a maximumdiameter, then the wash zone occupies the entire length of the filterdrum. When one end plate has a maximum diameter while the other one hasa minimum diameter, then only one-half of the drum length is used forwashing. Such an arrangement may then be necessary, when contact betweenthe washing liquid and the liquor of the suspension inside the productis to be avoided.

According to FIG. 2 the washing liquid is pumped into the wash zonethrough the central pipe 47 and through a flexible, longitudinallyadjustable hose 54 across the lower end plate 52 into the wash zone. Thepressure of the washing liquid must be greater than the pressure atwhich the suspension is fed into the annular chamber 25 (FIG. l). Thewashing liquid traverses the wall 29 of the inner drum 27 and penetratesinto the product from where it escapes again through the wall 28 of theouter filter drum and above the wash zone also through the wall 29 ofthe inner filter drum. The liquid filtered through the inner drum wall23 above the wash zone traverses the wash zone via the sealed centraltube 53, discharging from there together with the other filtrate throughthe inner hollow shaft l3 (FIG.

Where it is desirable, it is also possible to alternatingly arrangeseveral wash zones and filtering zones over the length of the drums.These wash zones would be supplied with pressurized washing liquideither via a common central pipe, or via several concentrically arrangedseparate pipes, or also via a hollow upper inner shaft 33.

It should be understood that, although the example chosen for theillustration of the invention shows cylindrical filtering drums, thedrum walls could also be other than cylindrical. For instance, they mayalso be in the form of other rotationally symmetric bodies or acombination thereof. Thus, an upwardly narrowing frustum shape of theouter drum 28 would give a filter chamber 25 of progressively decreasingcross section. If the decrease corresponds to the progressive reductionin product volume, a constant axial speed of product advance isobtained. When both filter surfaces are tapered, but at differentangles, one obtains an annular chamber whose cross section is variablein the axial direction, thereby permitting adjustment of the shearaction and viscosity under turbulence to optimal values, whenthixotropic substances are processed.

What is claimed is:

1. A pressure-filtering apparatus for the separation of a suspensioninto dried solid product and liquor in a continuous operation, theapparatus comprising:

a. a filter chamber defined by perforated walls in an inner and outerfilter drum, said drums being concentrically arranged;

b. a feed connection and a discharge connection communicating with saidfilter chamber;

c. a hollow inner shaft extending from said inner drum, said hollowinner shaft being adapted to permit discharge of liquor collectinginside said inner drum;

d. first and second hollow outer shafts extending from said outer drum,said inner shaft being concentrically disposed within said first outershaft to form an annular channel, said annular channel and said secondhollow outer shaft each effecting communication between said filterchamber and one of said connections;

e. means for moving said drums relative to one another thereby creatingturbulence and a mixing action in the suspension while the liquor isbeing filtered out under pressure.

2. An apparatus as defined in claim 1, further comprising:

means for washing the product inside the filter chamber by forcingpressurized washing liquid through one of the filtering walls;

the filter chamber including a wash zone occupying a distinct portion ofthe chamber, the washing means being connected to the wash zone.

3. An apparatus as defined in claim 1, wherein said inner and outerfilter drum are coaxially journalled;

the feed connection includes a feed chamber communicating with thefilter chamber, the feed chamber being axially sealed off by means ofrotary seals;

the discharge connection including a discharge chamber alsocommunicating with the filter chamber and similarly sealed off by meansof rotary seals; and

the moving means being constituted by drive means arranged to rotate thefilter drums relative to one another.

4. An apparatus as defined in claim 3, wherein the inner filter drumincludes flanges attached to the drum ends and inner drum shaftscoaxially extending from each flange, at least one of these shafts beinghollow to permit discharge of the liquor collecting inside the innerfilter drum, both inner drum shafts being journalled at an axialdistance from their respective flanges, and one inner drum shaft endbeing connected to the drive means;

the outer one of the two filter drums includes likewise flanges attachedto the drum ends, said first and second outer drum shafts coaxiallyextending from each flange, the outer drum shafts being shorter than theinner drum shafts, both outer drum shafts being joumalled near theirrespective flanges so as to leave an axial space between them and theinner shaft journals on each side of the drums, these axial spaces beingpart of the feed chamber and discharge chamber, respectively; the shaftsand flanges of the drums being radially spaced from one another to forma continuous annular axial channel for the flow of material into and outof the filter chamber, respectively, as part of the feed and dischargeconnections; one outer drum shaft being connected to the drive means.

5. An apparatus as defined in claim 4, wherein the journals for theinner and outer drum shafts are contained in stationary journal housingson each side of the drums, one of the two journal housings enclosing thefeed chamber between the journals and a stationary feed pipe openinginto the feed chamber, the other journal housing enclosing a similardischarge chamber and stationary discharge pipe.

6. An apparatus as defined in claim 3, further comprising a gas-tightstationary casing surrounding the filter drums and carrying the drumjournals;

a gas-tight liquor-collecting receptacle connected to the casing so asto communicate with the outer liquor-discharge area surrounding theouter drum as well as with the inner liquor-discharge area inside theinner drum; and

a liquor-discharge conduit to continuously remove the accumulatedliquor.

7. An apparatus as defined in claim 6, wherein the axis of the eachfilter drum is vertical;

the casing includes a bottom panel which carries a journal housing withthe lower drum journals and the feed chamber arranged in the journalhousing;

the casing further includes an upper closure panel which carries asimilar journal housing with the upper drum journals and the dischargechamber arranged therein;

the liquor-collecting receptacle is mounted underneath the bottom panelof the casing so as to surround the lower journal housing, the liquordischarging from the outer filter drum passing through apertures in thebottom panel into the receptacle, while the liquor discharging from theinner filter drum passes via a sealed axial passageway from the insideof the drum across the lower journal housing into the same receptacle.

8. An apparatus as defined in claim 3, wherein the common length of theconcentric filter drums is subdivided into at least one filtering zoneand one wash zone of predetermined lengths, the wash zone being formedby two axially spaced transverse separation walls sealingly arranged inthe cavity of the inner drum;

the apparatus further includes means for supplying highly pressurizedwashing liquid to the wash zone, so as to force the washing liquidthrough the perforated wall of the inner filter drum into thepressurized product flowing through the filter chamber.

9. An apparatus as defined in claim 8, wherein the common length of theconcentric filter drums includes two filtering zones and an intermediatewash zone; the length and longitudinal location of the wash zone beingadjustable through axial repositioning of the transverse separationwalls; the interior liquor discharge areas of the inner filter drum inthe filtering zone being connected to one another by means of a sealedduct extending across the wash zone.

10. An apparatus as defined in claim 8, wherein the common length of theconcentric filter drums includes several filtering zones and severalwash zones in longitudinal alternation.

11. An apparatus as defined in claim 8, wherein the washing liquidsupply means includes a supply pipe sealingly extending from the outsideof the drums into the washing zone along the drum axis.

12. An apparatus as defined in claim 8, wherein the wall of the innerfilter drum has an inner diameter which increases in a taper from thelongitudinal midpoint of the drum to both drum ends; and

the transverse separation walls forming the wash zone are axiallypositioned by clamping them .against oppositely tapering drum wallportions.

13. A method of pressure-filtering a suspension so as to separate itinto dried solid product and liquor comprising the steps of:

pressurizing the suspension between two opposite perforated filteringwalls of a filter chamber;

moving the two filtering walls in opposite directions to one another ata substantially constant distance from one another so as to create ashearing action and turbulence-induced mixing action in the suspension,while the liquor is forcibly filtered from it.

14. A filtering method as defined in claim 13, further comprising thestep of continuously feeding a pressurized suspension into the filterchamber on one of its extremities; and

continuously discharging the dried product cake from the filter chamberon its opposite extremity.

15. A filtering method as defined in claim 14, further comprising thestep of continuously washing the product during filtering by forciblyinjecting washing liquid into a portion of the filtering chamber.

16. A filtering method as defined in claim 14!, further comprising thestep of recirculating at least a portion of the continuously dischargedproduct cake into the filter chamber; and

washing the product cake to be recirculated before it is re-introducedinto the filter chamber.

17. A pressure-filtering apparatus for the separation of a suspensioninto dried solid product and liquor in a continuous operation, theapparatus comprising:

a. a filter chamber defined by perforated walls in an inner and outerfilter drum, said drums being concentrically arranged;

b. a feed connection and a discharge connection communicating with saidfilter chamber;

c. a liquor outlet means in the apparatus;

d. a first drive means coupled with said inner drum for rotating saidinner drum in one direction;

e. a second drive means coupled with said outer drum for rotating saidouter drum in a direction opposite to the direction of rotation of saidinner drum; thereby creating turbulence and a mixing action in thesuspension, while the liquor is being filtered out under pressure.

2. An apparatus as defined in claim 1, further comprising: means forwashing the product inside the filter chamber by forcing pressurizedwashing liquid through one of the filtering walls; the filter chamberincluding a wash zone occupying a distinct portion of the chamber, thewashing means being connected to the wash zone.
 3. An apparatus asdefined in claim 1, wherein said inner and outer filter drum arecoaxially journalled; the feed connection includes a feed chambercommunicating with the filter chamber, the feed chamber being axiallysealed off by means of rotary seals; the discharge connection includinga discharge chamber also communicating with the filter chamber andsimilarly sealed off by means of rotary seals; and the moving meansbeing constituted by drive means arranged to rotate the filter drumsrelative to one another.
 4. An apparatus as defined in claim 3, whereinthe inner filter drum includes flanges attached to the drum ends andinner drum shafts coaxially extending from each flange, at least one ofthese shafts being Hollow to permit discharge of the liquor collectinginside the inner filter drum, both inner drum shafts being journalled atan axial distance from their respective flanges, and one inner drumshaft end being connected to the drive means; the outer one of the twofilter drums includes likewise flanges attached to the drum ends, saidfirst and second outer drum shafts coaxially extending from each flange,the outer drum shafts being shorter than the inner drum shafts, bothouter drum shafts being journalled near their respective flanges so asto leave an axial space between them and the inner shaft journals oneach side of the drums, these axial spaces being part of the feedchamber and discharge chamber, respectively; the shafts and flanges ofthe drums being radially spaced from one another to form a continuousannular axial channel for the flow of material into and out of thefilter chamber, respectively, as part of the feed and dischargeconnections; one outer drum shaft being connected to the drive means. 5.An apparatus as defined in claim 4, wherein the journals for the innerand outer drum shafts are contained in stationary journal housings oneach side of the drums, one of the two journal housings enclosing thefeed chamber between the journals and a stationary feed pipe openinginto the feed chamber, the other journal housing enclosing a similardischarge chamber and stationary discharge pipe.
 6. An apparatus asdefined in claim 3, further comprising a gas-tight stationary casingsurrounding the filter drums and carrying the drum journals; a gas-tightliquor-collecting receptacle connected to the casing so as tocommunicate with the outer liquor-discharge area surrounding the outerdrum as well as with the inner liquor-discharge area inside the innerdrum; and a liquor-discharge conduit to continuously remove theaccumulated liquor.
 7. An apparatus as defined in claim 6, wherein theaxis of the each filter drum is vertical; the casing includes a bottompanel which carries a journal housing with the lower drum journals andthe feed chamber arranged in the journal housing; the casing furtherincludes an upper closure panel which carries a similar journal housingwith the upper drum journals and the discharge chamber arranged therein;the liquor-collecting receptacle is mounted underneath the bottom panelof the casing so as to surround the lower journal housing, the liquordischarging from the outer filter drum passing through apertures in thebottom panel into the receptacle, while the liquor discharging from theinner filter drum passes via a sealed axial passageway from the insideof the drum across the lower journal housing into the same receptacle.8. An apparatus as defined in claim 3, wherein the common length of theconcentric filter drums is subdivided into at least one filtering zoneand one wash zone of predetermined lengths, the wash zone being formedby two axially spaced transverse separation walls sealingly arranged inthe cavity of the inner drum; the apparatus further includes means forsupplying highly pressurized washing liquid to the wash zone, so as toforce the washing liquid through the perforated wall of the inner filterdrum into the pressurized product flowing through the filter chamber. 9.An apparatus as defined in claim 8, wherein the common length of theconcentric filter drums includes two filtering zones and an intermediatewash zone; the length and longitudinal location of the wash zone beingadjustable through axial repositioning of the transverse separationwalls; the interior liquor discharge areas of the inner filter drum inthe filtering zone being connected to one another by means of a sealedduct extending across the wash zone.
 10. An apparatus as defined inclaim 8, wherein the common length of the concentric filter drumsincludes several filtering zones and several wash zones in longitudinalalternation.
 11. An apparatus as defined in Claim 8, wherein the washingliquid supply means includes a supply pipe sealingly extending from theoutside of the drums into the washing zone along the drum axis.
 12. Anapparatus as defined in claim 8, wherein the wall of the inner filterdrum has an inner diameter which increases in a taper from thelongitudinal midpoint of the drum to both drum ends; and the transverseseparation walls forming the wash zone are axially positioned byclamping them against oppositely tapering drum wall portions.
 13. Amethod of pressure-filtering a suspension so as to separate it intodried solid product and liquor comprising the steps of: pressurizing thesuspension between two opposite perforated filtering walls of a filterchamber; moving the two filtering walls in opposite directions to oneanother at a substantially constant distance from one another so as tocreate a shearing action and turbulence-induced mixing action in thesuspension, while the liquor is forcibly filtered from it.
 14. Afiltering method as defined in claim 13, further comprising the step ofcontinuously feeding a pressurized suspension into the filter chamber onone of its extremities; and continuously discharging the dried productcake from the filter chamber on its opposite extremity.
 15. A filteringmethod as defined in claim 14, further comprising the step ofcontinuously washing the product during filtering by forcibly injectingwashing liquid into a portion of the filtering chamber.
 16. A filteringmethod as defined in claim 14, further comprising the step ofrecirculating at least a portion of the continuously discharged productcake into the filter chamber; and washing the product cake to berecirculated before it is re-introduced into the filter chamber.
 17. Apressure-filtering apparatus for the separation of a suspension intodried solid product and liquor in a continuous operation, the apparatuscomprising: a. a filter chamber defined by perforated walls in an innerand outer filter drum, said drums being concentrically arranged; b. afeed connection and a discharge connection communicating with saidfilter chamber; c. a liquor outlet means in the apparatus; d. a firstdrive means coupled with said inner drum for rotating said inner drum inone direction; e. a second drive means coupled with said outer drum forrotating said outer drum in a direction opposite to the direction ofrotation of said inner drum; thereby creating turbulence and a mixingaction in the suspension, while the liquor is being filtered out underpressure.